The disclosure generally relates to CMOS (complementary metal-oxide-semiconductor) image sensors employing pixels designs that utilize source followers for isolation devices, and more specifically relates to a CMOS image sensor that reduces the dependency of a pixel's output level on variations in the threshold voltage of the source follower.
An image sensor is a device that converts a visual image to an electric signal, such as those commonly found in digital cameras and other imaging devices. An active pixel sensor (APS) is an image sensor consisting of an integrated circuit containing an array of pixel sensors, and are produced by a CMOS process (hence the term “CMOS image sensors”), and are emerging as an inexpensive alternative to charge couple devices (CCDs).
Source followers are used for isolation devices in CMOS imaging sensors. An illustrative implementation of a CMOS imaging sensor typically includes a photo diode (PD) in which incident light causes the generation of minority carriers, and a reset transistor that performs the function of resetting the photo diode to the supply voltage Vdd. The signal which is the difference between the reset voltage and the voltage on the photo diode is applied to the gate of a source follower. The signal at the source of the source follower is then transmitted to a row select transistor. Accordingly, the source follower is used to drive the analog signal from the individual pixel cells out onto a shared column line. However, the operation of the pixel is strongly affected by the variations and tolerance of the threshold voltage Vt associated with the source follower. The threshold voltage Vt is subject to variations, e.g., as function of temperature and process parameters.
Accordingly, a need exists for a solution that can address unwanted variations in the output voltage of an active pixel sensor (APS).